1. Field of the Invention
The present invention relates to a gas sensor for detecting the concentration of a component gas contained in a gas to be measured, such as exhaust gas from an internal combustion engine.
2. Description of the Related Art
Conventionally, gas sensors are used for measuring the concentration of a component gas contained in a gas to be measured. For example, Japanese Patent Application Laid-Open (kokai) Nos. 9-196885 and 11-295263 disclose oxygen sensors having a cylindrical element. Further, another oxygen sensor having a cylindrical element is disclosed in Japanese Patent Application No. 11-228322. Meanwhile, Japanese Patent Application Laid-Open (kokai) No. 9-127050 discloses an oxygen sensor having a strip-shaped element.
In order to dispose an element for detecting a component gas in a path along which a gas to be measured flows, the above-mentioned conventional gas sensors are configured, for example, as shown in FIG. 1. Specifically, the element is disposed in a hollow portion extending through a metallic shell, and an inorganic powder, such as talc or a ceramic powder, is filled into a gap between the metallic shell and the element so as to hermetically seal the metallic shell and the element against each other. Particularly, when high gastightness must be established, an upper portion of the metallic shell is caulked so as to compress the filling inorganic powder from above to thereby enhance gastightness of the inorganic powder.
A conventional process for filling the inorganic powder into the gap between the element and the metallic shell will next be described with reference to FIG. 1. First, talc having a grain size of 5 xcexcm to 50 xcexcm is prepared. Water glass is mixed into the talc in an amount of 4 parts by weight per 100 parts by weight of talc. The resultant mixed powder is formed under pressure into a sheet. The thus-formed sheet is pulverized and sieved to obtain secondary particles having a particle size of 300 xcexcm to 800 xcexcm. The thus-obtained powder is placed into a die, and then compacted into the form of a ring. The ring-shaped compact is inserted into the gap between the element and the metallic shell. The inserted compact is crushed from above by means of a hydraulic press to thereby fill the gap with the inorganic powder. Then, a sleeve of alumina ceramic is inserted from above and placed onto the inorganic powder. An upper portion of the metallic shell is caulked so as to compress the inorganic powder via the sleeve, to thereby enhance gastightness of the inorganic powder.
The reason why water glass is mixed into the inorganic powder in the above-described process is that compressibility of the inorganic powder can be improved by the mixed water glass. As described in Japanese Patent Application No. 11-123122, when the water glass content is set to 2 to 7 parts by weight per 100 parts by weight of the organic powder, the resulting mixed powder exhibits improved workability when compacting into a ring. Also, when the ring is built into a gas sensor, the ring can be compressed at a high compression rate, so that the gas sensor thus assembled has a high degree of gastightness.
However, studies carried out by the present inventors reveal that when talc containing water glass is used as described above, sufficient gastightness cannot be maintained for a long period of time in an environment of 600xc2x0 C. or higher. That is, when water glass is contained in talc, the properties of the water glass change at temperatures higher than 600xc2x0 C., resulting in grain growth of the talc. Thus, high powder fluidity peculiar to talc is lost, resulting in impaired gastightness.
An object of the present invention is to solve the above-mentioned problems in a conventional gas sensor and to provide a gas sensor, in which an inorganic powder is filled into a gap between a metallic shell and a sensing element disposed in the metallic shell in order to establish a hermetic seal between the sensing element and the metallic shell, and which can maintain the established hermetic seal even when the gas sensor is used in an environment of 600xc2x0 C. or higher.
The above object, based on studies which the present inventors conducted on properties of an inorganic powder, has been achieved by providing a gas sensor comprising an inorganic powder which does not have an exothermic or endothermic peak within a temperature range up to 700xc2x0 C. when the inorganic powder is subjected to differential thermal analysis. If the inorganic powder does not produce an exothermic or endothermic peak when subjected to differential thermal analysis within a temperature range up to 700xc2x0 C., grains of the inorganic powder do not grow within a temperature range up to 700xc2x0 C. Thus, the gas sensor can maintain sufficient gastightness even in a working environment of 600xc2x0 C. or higher.
The present invention further provides a gas sensor comprising an inorganic powder whose weight loss caused by heating from room temperature to 700xc2x0 C. when subjected to differential thermal analysis is not greater than 0.5%. When the weight loss of the inorganic powder measured at 700xc2x0 C. is not greater than 0.5%, grains of the inorganic powder do not grow even in a working environment of 600xc2x0 C. or higher, so that the gas sensor can maintain sufficient gastightness even in such a high temperature working environment.
The present invention still further provides a gas sensor comprising an inorganic powder whose specific surface area changes 19% or less (absolute value) during heat treatment performed at 700xc2x0 C. for 24 hours. Hereinafter, the absolute value of a change in specific surface area is also called the xe2x80x9crate of change in specific surface area.xe2x80x9d When the rate of change in specific surface area is not greater than 19%, it is considered that grains of the inorganic powder do not grow even in a working environment of 600xc2x0 C. or higher. Thus, the gas sensor can maintain sufficient gastightness.
Preferably, the inorganic powder is insulative and, in particular, contains SiO2 and MgO such that the total weight of SiO2 and MgO accounts for not less than 98 wt % of the weight of the inorganic powder. By using the inorganic powder, the gas sensor can favorably maintain gastightness. Alternatively, the inorganic powder contains SiO2 and Al2O3 such that the total weight of SiO2 and Al2O3 accounts for not less than 98 wt % of the weight of the inorganic powder. These inorganic powders are inexpensive and exhibit excellent compressibility so as to impart a hermetically sealed condition to the gas sensor.
Since the above-mentioned inorganic powders contain substantially no water glass, in general, it is difficult to obtain secondary particles thereof having a particle size convenient for compacting into a ring. Therefore, it is preferred to pulverize material stone of talc (i.e., raw talc from a mine) into particles having a particle size convenient for compacting into a ring, and to compact the particles into a ring. In the case where particles formed directly into a ring shape are produced from a material stone of talc by pulverization, pulverization is preferably performed such that the resultant particles have a particle size of 400 xcexcm to 600 xcexcm, because particles having a particle size of 400 xcexcm to 600 xcexcm flow smoothly into a die and exhibit good formability.
Further, in the case in which the inorganic powder as described above is charged into a gap between the metallic shell and the element, a portion of the metallic shell is caulked so as to compress the inorganic powder, whereby the gap between the metallic shell and the element can be properly filled with the inorganic powder, and can be sealed hermetically.